This invention relates to holographic scanning, and more particularly to scanning a bar code symbol.
One use of scanning, and particularly laser scanning, is for reading a bar code symbol, which is a symbol with regions having different light reflective properties. One form of bar code symbol consists of elements which are in a series of bars and spaces that are typically rectangular in shape with a variety of possible widths. The specific arrangement of elements defines the characters represented according to a set of rules and definitions specified by the code or symbology used. One example of a symbology is the Uniform Product Code found on many food products.
In known scanning systems, a light beam is directed toward a target that includes a bar code symbol on the surface. The scanning functions by repetitively sweeping the light beam in a line or series of lines across the symbol. Scanning systems also include a sensor or photodetector which detects light reflected from the symbol. A portion of the reflected light which is reflected off the symbol is detected and converted into an electrical signal, and electronic circuitry or software decodes the electric signal into a digital representation of the data represented by the symbol that has been scanned.
A diffraction grating element can be used to direct a laser beam for scanning purposes. One scanning technique includes use of a laser beam directed at a rotating disc with a diffraction grating, also called a hologon. The grating can be produced optically, but for higher visibility can be computer generated. The beam is diffracted and forms a scan line that moves linearly as the hologon is rotated by a motor. If the entire disc consists of one type of pattern, or if there are multiple sectors with the same pattern, the scan line proceeds from a beginning point to a termination point, then returns to the beginning point before the next scan. This is called unidirectional scanning.